| Tunnels and bridges |
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| Building for travel |
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| The longest road tunnel in the world is in Norway. It is 24.5 Km long. |
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| Tunnels |
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| People cross mountains by car or train and they cross water by boat but these journeys take time. |
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| Tunnels can go under mountains and under water. |
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| They make journeys much quicker. |
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| A tunnel is an enormous tube and a tube is a very strong shape. |
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| When engineers build a tunnel, they make a tube shape in the ground. |
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| If the tunnel is not deep the work is easier. Engineers use two cut and cover methods. |
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| Method 1 |
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| First the trench is dug by diggers. |
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| Then tubes are carefully lowered by cranes. |
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| Finally the tubes are covered with earth by bulldozers. |
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| Method 2 |
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| First the walls are made in the ground with concrete. |
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| Then the roof is made and covered with earth. |
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| Finally, the tunnel is dug machines. |
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| If the tunnel is deep the engineers must use another method. |
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| They use an enormous machine to bore into the earth and rock. |
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| It is called a Tunnel Boring Machine (TBM). |
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| It cuts rock and clears it away at the same time. |
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| It can cut and clear a 25-30 metre length of tunnel each day. |
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| Cutting edges turn and break the rock into pieces. |
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| The rock pieces are carried away by a moving belt. |
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| Strong, thick, rods push the machine forwards. |
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| The Channel Tunnel joins England and France. |
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| It is a railway tunnel and it goes under the sea. |
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| Engineers used four TBMs at the same time to cut the tunnel. |
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| Bridges |
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| People have built bridges across rivers for thousands of years. |
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| There are many different kinds of bridge. |
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| beam bridge |
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| beam - the weight of the bridge - piers |
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| piers support the weight of the bridge. |
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| The beam bridge is the simplest kind of bridge. |
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| A log or a plank across a stream is a short beam bridge. |
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| A long beam bridge is supported by piers at each end |
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| This kind of bridge cannot cross a very wide river. |
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| Arch bridge |
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| the weight of the bridge - arch |
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| The shape of the arch helps to support a longer, heavier bridge. |
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| An arch can support a lot of weight so it hold up a longer bridge. |
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| Some bridges are made of more than one arch. |
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| Bridges made by people long ago often had many arches. |
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| The Bridge of 33 Arches in Iran is four hundred years old. |
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| This fascinating bridge has arches on two levels. |
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| In summer, it is full of visitors and delighted children paddle in the shallow water underneath. |
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| Suspension bridge |
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| cables - the weight of the bridge - high towers support long cables |
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| Strong cables can support a very long bridge. |
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| This kind of bridge can be even longer. |
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| The bridge is supported by strong cables. |
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| The cables are supported by tall towers. |
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| Suspension bridges are designed by engineers to cross wide rivers and sea channels. |
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| Listen 5 times |
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